1. Field of the Invention
This invention relates to photo-ionization detectors that use ion mobility spectrometry, ionization potential discrimination, and/or chemical filtering to detect, identify, and measure quantities of selected gases.
2. Description of Related Art
Conventionally, photo-ionization detectors (PIDs) measure the concentration of ionizable gases in a sample by measuring the number of ions created when UV light passes through the sample. Generally, PIDs perform xe2x80x9cbroadbandxe2x80x9d measurements that do not provide specific information that identifies the particular gas in the sample because the UV light ionizes all types of gases having ionization potentials below the maximum photon energy of the UV light and all of the ions are measured as a group. However, co-owned Pat. App. Ser. No. 09/330,522, now U.S. Pat. No. 6,320,388, issued on Nov. 20, 2001 , entitled xe2x80x9cMultiple Channel Photo-Ionization Detector for Simultaneous and Selective Measurement of Volatile Organic Compoundsxe2x80x9d, which is hereby incorporated by reference in its entirety, describes a PID employing a UV lamp having separate window sections that pass UV light with different energy spectra. Separate measurements of ion currents caused by the different UV light spectra can identify a range for the ionization potential of gases in a sample, and the identified ionization potential indicates chemical composition of the ionizable gas. However, gases having nearly identical ionization potentials may be difficult to distinguish using such techniques.
Ion mobility spectrometry (IMS) distinguishes compounds by gas phase ion mobilities. Conventionally, for IMS, a radioactive source such as Ni-63 ionizes molecules in a sample gas, and an electric field in a drift tube causes the ions to travel down the drift tube against the flow of a drift gas. Different types of ions typically reach the end of the drift tube at different times depending, for example, on the mass, the size, and the charge of the ion. A collector electrode at the end of the drift tube collects the ions which thereby generate one or more current pulses. Specific types of ions from the sample gas can be identified from the time periods measured for the ions to travel the length of the drift tube.
The use of a radioactive source, which requires licensing and special waste disposal, limits the acceptance of ion mobility spectrometry in commercial products. Further, radioactive sources tend to ionize a large class of compounds including common components of air such as oxygen, nitrogen, and water vapor. These ions can in turn react with other molecules and ions in the sample to generate a relatively large number of ion species. In general, the ion species have different mobilities and reach the collector at different times. However, the current pulses corresponding to the different ion mobilities can overlap, making difficult the discrimination or identification of a particular ion type.
A further problem in discriminating the various ion signals is non-uniformity of the electric field in the drift tube. In particular, a conventional drift tube includes a set of conducting rings that are in the walls of a cylindrical tube. Each ring has a different voltage level so that an electric field in the drift tube is directed from the rings at higher voltages toward the rings at lower voltages. With this configuration, the electric field is relatively weak along an axis that passes through the centers of the rings and increases radially toward the rings. Accordingly, ions traveling near the axis experience the weaker electric field and correspondingly have a lower average drift velocity. Ions traveling near the walls of the drift tube thus reach the collector electrode before ions traveling near the axis of the drift tube, and the signal peak corresponding to a particular type of ion is spread out in time by at least the difference in the travel time that the non-uniform electric field causes. Also, the electric field in each region that is surrounded by a ring has a relatively weak electric field because the surrounding ring has a uniform electric potential, while regions between the rings have a relatively strong electric field caused by the drop in electric potential between the rings. This causes axial variations in the electric field. Both the radial and axial variation in the electric fields broaden signal peaks and make different types of ions more difficult to distinguish.
In accordance with an embodiment of the invention, a photo-ionization detector (PID) uses one or more UV lamps to ionize a gas for an ion mobility measurement. Each lamp or window zone of a lamp produces light having a different UV spectrum and ionizes different sets of ionizable gas. Ion mobility measurements using the different UV spectra provide quantitative information about the ion types. Thus, this PID can distinguish different gases based on the ionization potential and ion mobility. The PID can identify specific chemicals in trace amounts. Accordingly, unlike conventional PID technology, which only provides broadband detection, PIDs disclosed here can not only realize broadband detection, but also selectively identify the presence and amount of specific gaseous chemicals.
From another perspective, embodiments of the invention provide an improved ion mobility spectrometer that employs one or more electrodeless UV lamps and does not require a radioactive element UV photo-ionization generates fewer ion species from a sample than would chemical ionization by a radioactive source and is easily tunable for selection of a particular ionization potential. The UV lamps are also more convenient to handle and use than is a radioactive source.
A further improvement of ion mobility spectrometers arises in embodiments of the invention that employ electrode configurations yielding a more uniform electric field in a drift tube. One such drift tube uses mesh plate electrodes rather than conventional hollow cylindrical electrodes, to form the electric drift field. With the mesh plate electrodes, the transformation efficiency of voltage to intensity of electric filed is much higher than that in the conventional configuration because in the new configuration the electric field is between the electrodes, rather than beside the electrodes as in the conventional configuration. The mesh electrodes reduce the radial non-uniformity in electric fields in drift tubes. The mesh electrodes also reduce the voltage required for a suitable electric field in a drift tube and hence benefit portable detectors.
In addition, the electrodes are very thin (for example, less than 1 mm) to improve axial uniformity of the electric field. In conventional drift tubes, relatively thick cylindrical electrodes cause an electric field across the thickness of each electrode to be weak when compared to the electric field between adjacent electrodes. The electric field is thus more uniform both radially and axially than the electric field in the traditional drift tubes, and the more uniform electric field reduces the total peak broadening. As a result, the detector has better resolution of the signal peaks and improved selectivity. Additionally, the reduction of peak broadening increases the peak heights of the signals and thereby improves sensitivity.
In accordance with another aspect of the invention, a tunable UV spectrum permits discrimination of chemical species according to differences in ionization potentials. One method for producing tunable photo-ionization uses multiple photo-ionization lamps with different maximum photon energies, for example, four UV lamps with maximum photon energies of 8.4, 9.8, 10.6, and 11.7 eV, respectively. Another method of producing tunable photo-ionization uses one UV lamp having multiple window zones and a zone selector. Each window zone passes a different spectrum of UV light. For example, the window of the lamp can include four different crystals having optical bandwidths with maximum photon energies of about 8.4 eV, 9.8 eV, 10.6 eV, and 11.7 eV, respectively. The zone selector chooses one zone of the window each time to allow a specified UV light spectrum to illuminate the sample and ionize the components of the sample. A sequence of ion mobility measurements uses different UV spectra to ionize the sample and measures drift times for the ions that each UV spectrum generates. Comparison of the peaks generated using different UV spectra can identify and approximate ionization potentials corresponding to specific peaks. In this manner, components having different ionization potentials are selectively ionized and discriminated.
In accordance with an aspect of the invention, a photo-ionization detector (PID) employs combinations of ion mobility spectrometry, ionization energy discrimination, and chemical filtering to identify the presence and quantity of specific gases. FIG. 1 shows a PID in accordance with an embodiment of this invention. The PID 10 includes an ionization chamber 14 and a drift tube 16. In an exemplary embodiment of the invention, ionization chamber 14 is a cylindrical about 20 mm in diameter and about 40 mm long, and drift tube 16 is also cylindrical with a diameter of about 20 mm but is about 100 mm long. The walls of ionization chamber 14 and drift tube 16 are made of a material such as Teflon, glass, or ceramics, which is resistant to chemical reactions with ions that are in ionization chamber 14 and drift tube 16 during operation of PID 10.